The XIfEvent function completes
only when the specified predicate procedure returns True for an event,
which indicates an event in the queue matches. XIfEvent flushes the output
buffer if it blocks waiting for additional events. XIfEvent removes the
matching event from the queue and copies the structure into the client-supplied
XEvent structure.

When the predicate procedure finds a match, XCheckIfEvent
copies the matched event into the client-supplied XEvent structure and
returns True. (This event is removed from the queue.) If the predicate
procedure finds no match, XCheckIfEvent returns False, and the output
buffer will have been flushed. All earlier events stored in the queue are
not discarded.

The XPeekIfEvent function returns only when the specified
predicate procedure returns True for an event. After the predicate procedure
finds a match, XPeekIfEvent copies the matched event into the client-supplied
XEvent structure without removing the event from the queue. XPeekIfEvent
flushes the output buffer if it blocks waiting for additional events.